1. Field of the Invention
This invention relates to novel, economical methods for determining the residual stress levels in machined, mechanically or thermally processed components and whether the stress is tensile or compressive. This invention also relates to a method of modeling of a component residual stress from production of raw billet through to a finished component.
2. Background of the Technology
Machining is the principal manufacturing process in the world with some 10-15% of the value of all goods being attributed either directly or indirectly to machining [Merchant, 1999]. However, with today's economic climate demanding products to be manufactured at increasingly reduced cost, machining cycle times have dramatically dropped. The use of high speed machining (HSM) techniques allows for high material removal rates, which in turn reduces machining cycle times [Dewes et al., 1995; Wyatt, 2002, Dewes et al., 1996; Smith et al., 1998]. Unfortunately, the capital cost of the equipment employed in HSM is high, which leaves machine shops that cannot make this type of investment in a precarious position with regard to the market place that they serve.
Conventional CNC machine shops are looking at increasing their machining parameters such as cutting speeds and feed rates etc. to improve productivity. However, these increases in speeds and feeds can lead to the machined surface, although visually acceptable, becoming abused. This abuse of the finished surface can lead to the component suffering from premature failure as a result of residual stresses that have been induced by the more abusive machining regime that the current market place has dictated.
Residual stress in a machined component is critical in determining both the wear and the fatigue characteristics of that component [Liu et al., 1984; Arndt, 1971]. It is not just the levels of stress that are critical but whether the stress is tensile or compressive. A machined surface that has tensile residual stress induced by the machining process would be prone to fatigue cracking whereas a machined surface with compressive residual stress would be resistant to fatigue cracking [Kalpakjian, 2001].
The experimental study of residual stresses associated with machined surfaces goes back at least fifty years [Henricksen, 1951]. Much of the early work was concerned with turning operations in an attempt to simplify the subsequent analysis of the mechanical state obtained. Extensive progress has since been made using a finite element method based approach to predicting residual stress distributions present after machining. Investigators have considered the highly important effects of sequential cutting action, as well as un-cut chip thickness in their simulations [Liu et al., 2000;Guo et al., 2000]. Those investigators have presented both residual stress profiles from the machined surface inwards, as well as the stress strain history of surface elements in their studies. Dominating are the effects of un-cut chip thickness in controlling the nature of the residual stress in the direction of the cut (tension versus compression).
In several of the experiments mentioned above, the investigators have assumed an initial state of zero stress and strain in the elements concerned. However, it is recognized that for heavy sections where there exist significantly differing cooling histories after heat treatment, for example thick aluminum alloy plate, the initial state of residual stress must be taken into account in subsequent modeling. Pechiney, has used such information in their simulations on the machining of billet intended for aerospace applications [Heymes et al., 1997]. Consequently, both material as well as final mechanical states must be taken into account if effects of subsequent behavior in monotonic or repeated loading are to be predicted.
Recent work on wrought aluminum alloys by SAAB, studied the effect of high speed machining on the fatigue strength of an aluminum workpiece [Ansell 1999]. The results were compared with those for samples that were prepared by conventional machining. The aluminum alloy was 7010-T7451, and panels were produced using different methods of machining. Fatigue test data showed that first a decrease in fatigue resistance was observed when the cutting speed increased above the conventional speed level, 100 m/min, but then an increase occurred when the cutting speed was raised towards 3,000 m/min. The minimum resistance appeared to be in the speed range of 200 to 500 m/min. The fatigue life of the specimens was also dependant upon the cutting mode employed. Climb milling was shown to give the largest reduction in fatigue resistance, while face cut milling seemed to develop a less serious reduction. Up-cut milling also gave a considerable reduction but less than that of climb milling. (See FIG. 1).
In reviewing the SAAB investigation [Blom et al., 2001], the Swedish Defense Research Agency presented an S-N curve indicating a serious reduction in fatigue life for various maximum stress levels. (See FIG. 2). Other researchers have suggested that high speed machining can be beneficial. For example, work at Sikorsky Aircraft supported by residual stress measurements at United Technologies Research Center [Fitzsimmons et al., 2000] suggests that near surface residual stresses in ground surfaces of a wrought titanium alloy can become tensional, whereas with the high speed machined condition chosen (not specified) the sub-surface residual stresses are predominantly compressive.
From an analysis of the work that has previously been undertaken there is still much that is not understood about how residual stresses are formed in high speed machining [Marchal, 2003;Peyronel, 2002]. It is felt that there may be windows of opportunity that high speed machining can exploit to give excellent fatigue life but that these areas have not yet been mapped, as shown in FIG. 1.
The current state of the art to measure residual stress is through the use of X-ray diffraction or neutron diffraction to measure residual stresses at a sub-microscopic level. Such methods measure how much the atomic spacing in the lattice structure of the material has changed and then calculates the residual stress from these measurements. However, these methods are both expensive and technically complicated to operate. There remains therefore a need for an accurate, easy to use, and economical method to measure residual stress.